NK2 files!!!
The number of times I've forgotten to back those up when moving a user onto a new machine!
One thing that annoys me with the way GMail handles this, is that my legitimate contacts get polluted with every person I've ever emailed. In GMail there's no way to turn the auto-add off, even though I've requested it for 3 years.
Thunderbird handles it well IMO (like Zimbra does at the moment) by having a separate folder where every emailed contact is added.
Keeping your real contacts separate from all those unsubscribe@ etc.
I hope this bug request doesn't override the current behaviour, but merely adds the options for people to change it
